1. Field of the Invention
The present invention relates in general to methods and apparatus for regulating and stabilizing the radiation intensity level of an X-ray source. The invention more specifically relates to such methods and apparatus that employ feedback circuitry for achieving such regulation and stabilization.
2. Description of the Prior Art
The radiation intensity of an X-ray source depends on the voltage potential between the anode and the cathode as well as the anode current of the X-ray tube. Thus, it is possible to control the radiation intensity level of the X-ray tube by controlling either the anode voltage or the anode current. It is not immaterial which one of these quantities one controls, because their effects on the characteristics of the radiation emitted by the tube are different. The anode voltage mainly controls the energy distribution of photons, i.e., the penetration of the radiation, whereas the anode current controls the number of photons emitted in a given time period.
Apart from the peak and average voltage levels, the wave forms of these voltages also have a considerable effect on the properties of the X-ray radiation. It is well known that in some applications of medical X-ray diagnostics considerable advantages are achieved if the anode voltage of the X-ray tube is as pure direct voltage as possible.
A practical method to make the anode voltage smooth, and both the anode and filament voltages adjustable has turned out to be a system in which the power supply voltage feeding the X-ray tube is at first modified to a crude DC voltage and is then modified with a controllable means into an adjustable DC voltage. This adjustable DC voltage is converted to an AC voltage of appropriate frequency and amplitude. The DC anode voltage for the tube is then formed from such AC voltage by means of a voltage multiplier comprising for example, capacitors and rectifying elements.
For forming the filament voltage, a system that is partially similar to the anode voltage supply may be used. The filament voltage supply differs from the anode voltage circuit in that the output voltage of the corresponding DC-AC converter is directly fed through an appropriate isolation transformer to the filament of the X-ray tube.
In the above-described system, both the anode voltage and the anode current (filament voltage) are set and adjusted through appropriate circuitry to make them remain constant, in principle. One possible way to stabilize the anode voltage is to use a single control loop where the feedback signal is taken directly from the anode voltage of the X-ray tube.
There are a few drawbacks in the arrangements described above are apparent when applying the system in practice. In the first place, the anode voltage and current do not stay constant even though the corresponding DC voltages feeding the DC-AC converters are stabilized. This is because, among other things, certain components between the regulating means and the controllable voltage X-ray tube are sensitive to heat. A second drawback is that when using feedback directly from the X-ray tube, the control loop must, because of stability, be set so slow that the supply frequency ripple contained by the crude DC voltage can still be detected in the high voltage. For the same reason (to maintain stability), the high voltage rise time during the switch-on of the device must be set too long to be favorable from the point of view of most applications.